simple answer and it works for me, very well too.
1. knock door
2. door is answered with flyer in hand simply say 'hello im doing a bit of canvassing for window cleaning in the area, would you be interested?' (watch body language and if they seem interested go for the kill)
3. they say 'yes i would be how much for a house this size?'
4. you reply 'it just depends on the amount of windows, we have no set prices, have you a conservatory out the back, do you mind if i have a wee look now and give you a quote?'
5 they reply 'sure go on ahead'
6. you take a wee look, dont rush it, take your time to see what access is like etc then knock the door again and give them your price, dont be shy about asking a bit higher then go for it worst you can get is a flat NO, if they have said yes in the first place they wont be too hard to get a final yes out of, after that you just find out how often and how they are going to pay you etc.
Simple method.